1. Field of the Invention
The present invention relates to welding members such as of a welding backing member, a coated welding rod, a welding bevel and the like, and in particular to welding members suited for welding in highly humid environment, during rainfall or in water. The invention also relates to a wet underwater welding method which enables welding to be effectively effected in a highly humid or underwater atmosphere.
The welding member and underwater welding method according to the present invention can be suitably applied, for instance, to wet underwater welding for marine structures in respect of ocean jointing, underwater repairing and maintenance and the like. Those marine structures include for example petroleum production facilities, loading and unloading facilities, storage facilities, sea airports, sea stockpiling bases, bridges, submarine tunnels, electricity generation facilities of a wave-tide-temperature difference system, ocean corrals and the like.
In addition, the welding member and welding method of the invention are useful for land welding of such steel structures as of ships, bridges and the like.
2. Description of the Related Art
In wet underwater welding, a certain backing member has heretofore been used which is formed of the same material (common metal) as that employed in an area to be welded and welded integrally to that welding area. With respect to a backing member for use in wet underwater welding as being removable after welding, those researches thus far done are directed solely to a method in which a metal backing member made of steel is put to use.
On the other hand, a ceramic material for example is known for use as a backing member in land welding. This ceramic backing member is usually made to be of a short length and hence is used in plural array along a weldline. The backing member before use is usually dried in a drying oven.
The conventional backing members noted above are encountered with the following problems.
(1) In the Case of Use of a Common Metal for a Backing Member in Wet Underwater Welding:
Since the resultant backing member gets remained on a welded area, painting or other treatment cannot be applied to the surface of the welded area having such backing member superimposed thereon. A marine structure in particular poses the problem that gap corrosion occurs between the welded area and the backing member. In such instance, the gap corrosion should be prevented by disposing a sacrifice anode in proximity to the backing member and then by allowing the anode to be corroded.
Moreover, a welded joint with the backing member remaining thereon reveals a fatigue resistance of about 80% as against the counterpart of a completely melted uranami welded joint using a ceramic backing material or the like that permits removal after welding. In consequence, when care is taken of fatigue as to a starting material to be welded, the plate thickness is required to be large. This is responsible for added material cost and increased production cost.
(2) In the Case of Use of a Copper Backing Member in Wet Underwater Welding:
Even with use of a copper backing member which is removable after welding, underwater welding is possible. The finished back bead, however, involves rather a rough surface and thus results in reduced fatigue strength of the resultant welded area.
Additionally, since the copper backing member is provided therein with a conduit for flow of cooling water, the same is usually large in length and inflexible in nature. The copper backing member fails to favorably follow joint variance or deformation of two mating members to be butt-welded with the result that a melted metal intrudes into a gap between the backing member and the members to be welded. Thus, the resulting back bead is often susceptible to unacceptable shaping. In such case, strict precision is required in bringing the mating welding members into accurately butted relation to each other, coupled with extra time and cost in preparation for welding. The copper backing member is so structured as to have disposed therein a cooling water conduit as mentioned above.
Joining of copper backing members of a short length to each other results in complicated structure and inconvenient handling.
(3) In the Case of Use of a Seramic Backing Member in Common Use for Land Welding in Wet Underwater Welding:
A ceramic backing member obtained in commerce is used with one of a generally square shape like a plate of a short length placed in plural array. This backing arrangement is flexible as a whole, missible with a melted metal and good in back bead formation and therefore has been widely acceptable for land welding. However, this backing member when used in water becomes hygroscopic as it is porous, and a water content having pooled inside the backing member decomposes upon exposure to the heat arising from a welding arc, thereby generating a hydrogen gas in large quantities. This gas causes disturbed arc, ultimately making it impossible to conduct welding.
To overcome the problem with hygroscopicity discussed above, it is required that a dense backing member be produced with a composition of the backing member currently practiced in the art. To this end, calcination should be effected at an extremely high temperature with eventual need for use of expensive production equipment.
(4) A Backing Member for Land Welding:
When placed in humid condition or wetted with water, a ceramic backing member in common use for land welding absorbs water and generates a large amount of a hydrogen gas during welding as stated above in item (3), thus causing weld defects such as blowholes, cracks and the like. Therefore, the ceramic backing member is usually unsuitable for use in a highly humid or rainy environment. Even in in-plant welding, it needs to be dried in a drying oven prior to use. A burden of extra control cost is thus necessary for making the ceramic backing member free from being hygroscopic.
The ceramic backing member is not applicable in the case where joint is great at welding bevels. It is known that this joint mis-alignment height can be coped with by a backing method in which use is made of aparticulate flux employed in one-sided land welding, or by a backing method in which a strip of glass fiber is used. Because the particulate flux and glass fiber strip are of a hygroscopic nature, those known methods are unsuitable for use in a highly humid or underwater atmosphere. The backing members for welding use in such methods absorb moisture when placed in humid condition or wetted with water, ultimately generating a hydrogen gas in quantity during welding and involving weld defects such as blow holes, cracks and the like.
In order to prevent the particulate flux from becoming adversely hygroscopic, "a backing flux for one-sided welding and a method of producing the same" is disclosed in Japanese Patent Laid-Open No. 48-49. Namely, coal tar is added in an amount of 0.5-8% to flux components so as to achieve improved fixing between a melted metal and a particulate flux. Although this prior art method is effective in avoiding the flux from becoming hygroscopic while in storage, the flux is less repellent to water but rather hygroscopic on contact with water during rainfall or in watery condition. The flux cannot be used for welding unless it is dried.
The foregoing description is related to the problems with backing members. Further problems arise from welding bevels and welding rods in the case of welding in a highly humid, rainy or underwater environment. To be more specific, two members to be welded are generally processed at their bevels in advance in plants, whereas steel materials are not particularly treated in most cases. Thus, rust occurs at those bevels and needs to be removed in situ at the time of welding operation. Depending upon the kind of materials used, the bevels are often protected with a tape-like protector adhesively bonded thereover. This adds a process step of releasing the protector before welding operation.
Besides and particularly, as a result of the recent advent of large-scale structures fixed in water, large-scale structures installed to be floating in water and the like, a certain technique has enjoyed reputation which can effect welding of high quality even in the severest underwater environment. In order to ensure that high-quality welding be achieved in such a wet environment as in underwater welding, special facilities are required to be located to artificially define an ideal local environment surrounding a welding area.
This invites increased cost which creates an obstacle to practical application of the above stated technique.
Since two members to be welded gather rust due to moisture or humidity caused by condensation, meticulous attention has been paid to rust removal and rust prevention for example at their bevels. Furthermore, when these members become hygroscopic or absorbs moisture on contact with water, the resulting water content decomposes in the influence of the heat arising from a welding arc and hence generates a large amount of a hydrogen gas which in turn produces weld faults such as blow holes, fractures and the like.
It has been taken as absolutely necessary, in recent years, that a welding member should be maintained in proper or appropriate condition by removing foreign matter such as moisture or the like therefrom in such a manner that the resultant welded area can be rendered durable so as to obtain the finished product of improved quality. For example, a coated welding rod or a flux has been stored in a temperature- and humidity-constant chamber or in an inert gas atmosphere. The foreign matter present in the welding area has been removed by means of suitable physical and chemical methods combined together. As the hazard or damage of a breakage accident is greater which would be due to welding failure as in atomic power-related machinery and high-pressure gas machinery, the quality control is stricter with respect to welding operation.
For example, a coated welding rod for use in wet underwater welding is obtained in commerce which has been made proof to water on its outer surface by copper plating. In underwater repairing usually conducted, a commercially available welding rod for land welding has in many cases been used after being rendered watertight simply by winding a vinyl tape around an outer surface thereof. Additionally, Japanese Patent Laid-Open No. 1-233093 discloses a coated welding rod of a water-repellent type for use in atmospheric welding but not in underwater welding. This is a coated welding rod of a non- or low-hydrogen type obtained by coating a copper core wire use of a coating agent composed of silicone in an amount of 0.05-0.20% by weight and, as the balance, an arc stabilizer, a slug-forming agent, a deacidification agent, an organic matter and a fixing agent.
However, the copper-plated water-proof coated welding rod is costly, and once being subject to a flaw, the copper-plated film absorbs moisture thereat and loses a beneficial effect inherent to copper plating. In regard to the coated welding rod obtained commercially and wound with a vinyl tape, water tends to intrude into a gap between the tape and the rod, or the tape has a strong tendency to get damaged and moistened when the welding worker remains dived for an extended length of time and stores the welding rod in a storage kit, takes the rod out of the kit, or secures the rod to a welding holder. Lastly, the coated welding rod of a non- or low-hydrogen type containing a silicone-containing coating agent is a special one resulting from addition of silicone in the course of formation of the welding rod. In water, however, such welding rod is partially hygroscopic and totally inapplicable. The welding rod may be useful in a highly humid environment, but on contact with water such as rain or the like, it becomes moistened as it is insufficiently repellent to water and hence needs to be dried prior to use.